n6 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, igO2 1903. 



The perturbation in H which precedes this, is on the contrary, as already stated, well developed and 

 powerful southwards towards the equator. 



We may therefore safely assume that we here have two phenomena to be dealt with, one connec- 

 ted with the storm in the north, and before it an equatorial perturbation of a kind similar to that of the 

 26th January, 1903. 



The placing of the normal line on the magnetograms has occasioned no special difficulty. The 

 storms are fairly powerful and well-defined at all the stations with the exception of Christchurch and 

 Honolulu ; the perturbing force can therefore be taken out with very satisfactory accuracy. The following 

 circumstances are taken into consideration in the drawing of the normal line. In declination the condi- 

 tions are simple, as there the perturbation is of short duration. The quiet parts before and after the 

 perturbation are connected in such a manner that the form of the curve corresponds with that at the 

 same hour on the nearest calm days. The conditions in the horizontal intensity are somewhat more 

 difficult, as there, as we have said, there is a long perturbation in front of the one under consideration. 

 In this, judging from things in general, the curve for most of the stations is normal at about 3 h , and 

 for an hour afterwards. The absolute distance of the normal line from the base-line on the magneto- 

 gram will thereby be determined; and its further course is regulated by the nearest calm days. 



THE EQUATORIAL PERTURBATION. 



41. As early as I9 h , those little, sudden, very variable perturbations are noticed, which occur simul- 

 taneously all over the earth, and symmetrically as regards the magnetic axis. It will be seen from the 

 copies of the //-curve that the conditions at Dehra Dun, Batavia and Honolulu entirely correspond with 

 one another. The force is mainly directed northwards. The perturbation appears to be over at about 

 23 h i2 m . From 2i h 28 m to 23'', the force remains almost constant both in magnitude and direction. The 

 perturbing forces are calculated for 22 h , and the corresponding current-arrows are marked upon the chart. 



Current-Arrows for the 30th March, 1903, at 22 h . 



Fig- 53- 



